The present invention relates to a digital gate pulse generator for the control of thyristors, to static inverter and/or converter control with such a digital gate pulse generator, and to control and regulation of a motor drive, in particular of the synchronous motor type, embodying static control through such a digital gate pulse generator.
More specifically, the invention relates to a digital gate pulse generator which is particularly adapted to microprocessing, and which can be integrated in the microprocessing treatment of a regulation and control loop through an inverter, or converter system, as can be formed for speed and torque adjustment in a motor drive. The digital gate pulse generator according to the present invention is particularly applicable to a current-fed inverter and control of the thyristors thereof which are load commutated.
The invention lends itself particularly to thyristor control under varying frequency on the AC lines and is therefore well adapted to speed control of a synchronous motor drive as well as to power conversion through static-controlled frequency changers.
The invention will be described for illustration purpose in the context of a current-fed static-controlled synchronous motor drive. The digital pulse generator according to the invention is part of a control loop including the thyristors of the control system of the motor drive. In contrast to the voltage-fed type, in a current-fed motor drive system the amplitude and the frequency of the alternating current that excites the stator windings of the motor are the control parameters. Typically, the motor is energized by an inverter formed of a plurality of controllable electric valves or switching devices (e.g., thyristors) of the type having the ability to hold OFF forward voltage until turned ON by a pulse from a gate pulse generator. The amplitude of alternating current supplied to the motor can be regulated or controlled as desired by adjusting the average magnitude of voltage impressed on the DC side of the inverter, while the frequency of the current is controlled by appropriately varying the switching frequency of the thyristors of the inverter.
A variable speed motor drive involves frequency adjustment of the power supply to the motor in order to regulate speed. In such a motor drive, gating of the static switches which determine the motor power supply parameters at any speed must be operable at varying frequency and through a range extending down to zero motor speed.
In the prior art systems embodying thyristors which are naturally commutated, the firing angle of the thyristors is controlled by reference to a time wave related to the voltage applied to the main electrodes of the thyristor to be turned ON, at any given moment. The generation of such a time wave reference is disclosed, typically, in U.S. Pat. Nos. 4,173,722 and 4,028,609 of R. L. Detering. There, however, only small variations of frequency are contemplated. Therefore, a digital pulse generator uses a digital counter synchronized in phase and frequency with the fundamental voltage sine wave, and a time wave reference is derived with which to determine the firing angle in relation to a digital counter. A phaselocked loop provides compensation for any change in the fundamental frequency of the AC power supply.
When the motor drive includes a synchronous motor supplied by an inverter, variable speed control through firing angle adjustment of the inverter thyristor is directly related through the time wave reference to the voltage and frequency parameters of the rotating vector derived on the stator, or the rotor of the machine, since there is an inherent synchronism between the two under steady conditions of operation. Accordingly, it has been proposed to use the motor terminal voltage to synchronize the inverter triggering pulses. See H. Le-Huy, A. Jakubowicz and R. Perret, "A Self-Controlled Synchronous Motor Drive Using Terminal Voltage-Sensing," in 1980 IEEE, pp. 562-569. There, the polarity and zero-crossings of the machine voltages are detected by means of terminal sensors from which is derived a speed signal as well as a rotor position signal. Although this prior art technique has been intended to be used as part of a microprocessor-based control system, it lacks the advantage of an all-digital treatment of the information. In particular, three voltage signals are derived and a speed signal is obtained therefrom by a frequency multiplier, the operating frequency range depending upon a counter length, the frequency of a clock and a chosen multiplying factor. These four signals are used to provide a controllable delay angle between the inverter triggering pulse and the machine voltages. Based on linear digital ramp technique the angle control word issued by a microcomputer is converted to a corresponding delay angle. Typically, the 8-bit input control word used there contains mode operation as well as delay-angle information. These are typically defined by the first bit and the remaining 7 bits, respectively, as explained in H. Le-Huy, R. Perret and D. Roye, "Microprocessor Control of a Current-Fed Synchronous Motor Drive" in 1979 IAS 79:29A pp. 873-880. Pulse generation, which is to be implemented either by software or by hardware, is also shown to use a single processor and hardwired logic circuits to convert the CPU commands to appropriate triggering pulses for the converter.
Another approach to firing angle determination and implementation within a control system using microprocessor technique is found in "Digitally Controlled Thyristor Source" by Guy Olivier, V. R. Stefanovic and Mohamed Akhtar Jamil in IEEE 1979, Vol. 1ECI-26 No. 3, August 1979, pp. 185-191. There, the transfer characteristic of a sixpulse thyristor bridge is used and a linear transfer relation between the control signal and the output voltage is sought through a PROM containing a table of an arc cosine curve. The SCR firing pulses are delayed by a time period in accordance with a signal derived from a PI controller in a closed-loop, and the output of the PI controller is fed into the linearization PROM memory which gives a digital equivalent of the desired firing angle. Each time a zero-crossing is detected in the bridge input voltage, one of two counters is started counting up until its output becomes equal to or larger than the desired firing angle, and a firing signal is sent to a ring-counter. This technique is more like that with the aforementioned Detering patents since the problem involved here, with a constantly varying frequency, is not encountered.
Microprocessor-controlled speed regulation in motor drives have been used with motor drives. See for instance the following papers:
"Microprocessor-Controlled Fast-Response Speed Regulator For Thyristorized Reversible Regenerative DCM Drives" in IECI 78 Proceedings--Industrial Applications of Microprocessors, March 20-22, 1978, pp. 216-221 by Kenzo Kamiyama, Azusawa and Miyahara.
"Microprocessor Controlled Inverter-Fed Synchronous Motor Drive" by W. Richter, pp. 161-163, 2nd International Conference on Electrical Variable-Speed Drives, 25-27 September 1979, IEEE Power Division (London).
G. A. Tendulkar, "Measurement of Speed, Position and Acceleration of Electrical Drives in Microprocessor-Based Control System" pp. 171-175, 2nd International Conference IEEE, London, September 1979.
"Control of DC Drives by Microprocessors" by E. Schneider, pp. 603-608, IFAC Symposium on Control in Power Electronics and Electrical Drives by W. Leonhard, Dusseldorf, 1978.
"Design of An Optical Autodaptive Current Loop for DC Motor Realization with a Hybrid Device Including a Microprocessor" by A. Oumamar, T. P. Louis and A. El-Hefnawy, IFAC Symposium, Dusseldorf, 1978.
The Schneider and Kamiyama papers, for instance, are involved with DC motor drives. Therefore they do not call for a direct derivation of an electrical angle related to rotor positioning and independently of speed, e.g., of frequency.